Over the past two weeks, President Biden has been pilloried from all sides for his performance during the recent CNN presidential debate, perceived by many as weak and halting. And as his subsequent public appearances have done little to slow calls for him to bow out of the presidential race, a question has been on many voters’ minds: Is it possible to be too old to be president?

Biden and former President Donald Trump are the two oldest major-party candidates whom voters have ever encountered on their ballots. If Biden wins the election, he’ll be 82 come Inauguration Day. Trump would be 78.

And while age and mental acuity have a complicated relationship, the candidates’ ages have affected voters’ perceptions of how well they’d be able to do their jobs.

That was the case even before the debate: Only 15% of voters were at least “very confident” that Biden had the physical fitness required to be president, and 21% felt similarly about his mental fitness, according to an April Pew Research Center report. (Trump fared better, with 36% expressing that level of confidence in his physical fitness and 38% in his mental ability.)

The Founding Fathers certainly thought about presidential age centuries ago: It’s baked into Article 2 of the U.S. Constitution, which requires that, among other things, a president be at least 35 years old. But while there’s a limit on how young one can be to assume the office, there has never been a rule about how old a president could be.

Today, nearly 80% of U.S. adults surveyed support having upper age limits for federal elected officials, including the president, according to Pew. But such a limit would have never occurred to the architects of the U.S. government in the 18th century, constitutional experts say, and there are huge barriers to those rules changing anytime soon.

Age was but a number at the Constitutional Convention

The 1787 Constitutional Convention in Philadelphia was full of intense debates that shaped the Constitution and are memorable even now. It’s easy to assume that the age requirement was similarly the product of a lot of time and deliberation. But that wasn’t necessarily the case. After all, the Founders were busy constructing an entire government from scratch, says Julian Davis Mortenson, a constitutional law professor at the University of Michigan.

“There was literally an infinite array of things they had to resolve,” he says. Case in point: Should there be an executive branch in the first place? Would there be one single president or multiple leaders? And how would they be chosen?

“And there’s only so much bandwidth even over the course of a four- or five-month drafting process. Some of the things they thought about, they thought about a lot. And some of the things just didn’t really come up.”

When the Founding Fathers settled on the idea of a single, powerful executive, it was clear that the role required someone trustworthy and capable enough to take up the mantle, Mortenson says.

So how to screen for that? The Founders came up with formal requirements that, in their minds, correlated with those qualities, says Buckner F. Melton Jr., a history professor at Middle Georgia State University. They’d have to be a resident of the U.S. for at least 14 years to ensure they were familiar with the laws and customs of the country, and they’d have to be a natural-born citizen to protect the presidency from outside influence.

A minimum age limit was put into place because “age was the best corollary they had for sound judgment, maturity and what we might refer to as wisdom,” Melton says. And they picked 35 — slightly higher than the Senate requirement (30) and the House (25).

So if there was a floor, why wasn’t there a ceiling? According to all the convention documentation, the idea wasn’t floated at all, Melton says. He has a theory as to why: Life spans were much shorter back in that part of the 18th century, so the idea that someone could be in political office at an age when they might not be at their peak mental fitness probably did not occur to the Founders.

Presidential age and mental capacity became increasingly relevant

As time went on, industrial-era medical advances like antibiotics and antiseptics meant that people tended to die later than they once did, Melton says. And as presidents started to live longer — and to have disabling medical emergencies — the question of capacity to serve started to emerge.

President Woodrow Wilson, who had a stroke in 1919 at age 63 that “seriously compromised” his ability to work, became emblematic of that concern, Melton says. For the next year and a half of his presidency, Wilson was mostly bedridden, and his wife, Edith Wilson, and doctors played a large role in helping him with his presidential duties — all while the public was mostly left in the dark. (The extent to which they were assisting him with his presidential duties is the subject of some debate. Although Edith Wilson said in her autobiography that she “never made a single decision regarding the disposition of public affairs,” most historians agree she had a heavy hand in his decision-making.)

It became clear to those in Wilson’s Cabinet that he wouldn’t step aside from the role to let someone else, even his vice president, assume office, Mortenson says. And there wasn’t much that anyone could do about it — after all, the Constitution mentioned what would happen when presidents died but not what would happen when their ability to serve became greatly diminished.

The question would continue to arise over the decades to come, particularly when President Franklin D. Roosevelt’s health declined precipitously before he died in office in 1945, at age 63, and when President Dwight Eisenhower experienced a heart attack and other serious illnesses in the 1950s. But things reached a tipping point in 1963, when President John F. Kennedy was assassinated and Lyndon B. Johnson was sworn into office. There were concerns about the hardiness of the line of succession: At the time of the assassination, Johnson had experienced a near-fatal heart attack years earlier, and the speaker of the House and the president pro tempore of the Senate were 71 and 86, respectively.

“Once you have experience with concrete, real-world events prompting worry about something like what happens when the president can’t do his job, you get political responses,” Mortenson says. The response this time: the 25th Amendment, ratified by the states and adopted in 1967.

Among other things, the 25th Amendment lays out how a vice president would take over for a president unable to perform official duties. It also specifies the process through which a president unable to fulfill duties could be removed from office by the vice president, a majority of the Cabinet and a supermajority of Congress.

The removal portion of the amendment has never been invoked — even though this had reportedly been considered by at least some of President Ronald Reagan’s advisers during his second term due to his alleged declining mental state. (He announced his diagnosis of Alzheimer’s disease in 1994, five years after he left the presidency. At the time, he was the oldest president to leave the Oval Office, at 77 years.) The matter was also raised in the wake of the Jan. 6 insurrection; Sen. Chuck Schumer, D-N.Y., and Democrats from the House Judiciary Committee called for then-Vice President Mike Pence to invoke the 25th Amendment to strip Trump of his powers.

But this amendment is the closest we’ve come to politically addressing concerns about a president’s age and mental faculties, Mortenson says.

What would it take to install a presidential age limit?

Even though polls show widespread support for age limits, the obstacles to changing the requirements for becoming the president are considerable. For a rule change to stand up in courts, the Constitution itself would need to be amended, Mortensen says.

The bar for passing an amendment is notoriously high — it needs to clear a two-thirds vote in the House and Senate, or two-thirds of the states can petition Congress to call a constitutional convention. From there, three-fourths of the country’s state legislatures must ratify the amendment for it to become a part of the U.S. Constitution.

It seems unlikely that Congress would be amenable to making any changes to the presidential age requirement, Melton says, because serving as a representative or senator is often a stepping stone to the presidency.

“If Congress is going to be reluctant to impose age limits because they may see themselves as cutting themselves off from the White House by doing this, then a convention of the states will be the only way out,” he says. “And Congress has shown in the past that it is perfectly willing to throw up roadblocks to that process.”

It’s not just a question of politics — it’s also a matter of neuroscience. There’s the most basic question: What would the limit be? There’s not one set age at which a person’s cognitive abilities decline, and some people don’t decline much at all. Alternatively, others — including Reagan’s daughter — have proposed using some kind of cognitive test as a gauge.

“We would need to have to come up with some sort of good, empirical, verifiable, reliable tests on which there’s a really good, strong consensus to decide when someone would be too old,” Melton says. “The other thing you could do, of course, is just go with a bright line and say, after age 70, that’s it. We do that with retirements. Why not with the White House?”

Whatever the fix, voters seem to have come to a sort of consensus, he says. “While the country is horribly divided right now … there is a type of weird uniformity where everyone realizes, whatever your politics are, a presidential candidate can be too old.”